I'd suggest you just go for 3-4 models first as any higher number would be very hard to make balanced if we want them all to have different (but still similar) "core spells" (spells that they start with)
There are currently 2 core spells used in Warlock:
Scourge which works as a close range AoE spell that deals dmg to both you and ur opponent on use, but only knocks back ur opponent ofc, it's got a very low cooldown.
And the familiar
Fireball which works as your most basic projectile spell with medium damage and a relatively low cooldown compared to other spells
I would suggest maybe add an inherent movement spell (R-spell in warlock) dependent on the hero you choose to play with.
That would make the choice of hero much much MUCH more important than it currently is in warlock now and make it more strategic instead of just having a few builds be generally stronger than most no matter which model you choose to play with.
The scourge could have lots of different effects to make them varied.
Lets take the basic scourge first, 10 dmg and 300 range (just so we have a base)
A 2nd scourge could be 7 dmg and 400 range. 3 dmg reduced might sound like a bit too big of a nerf compared to the 100 range increase, but since it also means you'll deal 3 less selfdamage from the scourge, and the range is like superimportant when it comes to that spell, it's pretty fitting imo

A 3rd scourge could be that it does 6 dmg, 300 range and leave ur opponents movement slowed by 50% for 2 seconds making it a combostarter
A 4th could be that it does 8 dmg, 300 range, but summons a shield around ur hero for 3 seconds
And a 5th could deal 9 dmg, 300 range, and increase ur own MS by 20% for 5s or smth
Different fireball effects is a bit harder to come up with as it should still stay as the most basic projectile spell, and thats hard when you wanna make 4 different

But i have a few suggestions:
1st suggestion: A supersmall, low damage fireball with increasing size and damage depending on the distance it's traveled
2nd suggestion: A low dmg fireball which leaves a trail of fire behind that deal DPS to opponents and increase MS of allies
3rd suggestion: A low dmg fireball which slows ur opponents MS by 60% for 1.5s
4th suggestion: A medium dmg fireball which deals extra DPS + 20% MS slow for the same duration as the DPS(3-5 seconds?)
So we have 5 scourges and 5 fireballs(including the basic one) so we could make 5 pairs here now, but if we wanna make the movement spells be core spells aswell we need 5 of those first aswell.
The names are pretty hard to come up with as i feel they should fit the hero they would belong to, but the effects could be pretty similar to the ones you find on the R-spells in warlock:
Teleport(basic),
Multiport(2-step-teleport),
Swap(switches position between you and the object you hit),
Relocate(low range and low cooldown teleport by projectile like in swap) and
Thrust(charge towards where you clicked at very high speed knocking back whoever might cross ur way and stopping ur own movement in the process ofc)
Then we could combine:
Teleport with basic scourge and basic fireball(the basic hero^^)
Multiport with shielding scourge and expanding fireball
Swap with slowing scourge and the firetrail fireball
Relocate with 400 range scourge and 60% slow fireball
Thrust with 20% inc MS scourge and DPS fireball
And come up with 5 hero ideas that would fit those
I think we should start with coming up with spells if we want the heroes to fit the spells cause we can't do it the other way(making the spells fit the heroes) if we want a successfully balanced game
